The invention relates to an electric switch and more particularly to a four-break electric switch having two pairs of rotatably movable contacts and four fixed contacts and a contact suitable for use in such a switch.
Manually operated electric switches having ratings of up to several hundred amps are well known. Some such switches include an integral fuse element. Typically, the load contact, i.e. fixed contact, in such a switch is selectively engaged and disengaged by a movable blade, i.e. movable contact, that is cantilever supported in spaced relation to the fixed contact. Of course, to withstand high currents and effectively break the high currents, the contacts and the space in which the movable contacts move must be relatively large. To reduce the size of electric switches, it has been proposed to mount movable contacts to rotate about a central portion thereof to accomplish two breaks with two fixed contacts (one break at each end of the movable contact) in about the same space as one break is accomplished in the cantilever type of contact mounting. The word "break" as used herein refers to a location at which movable contacts can be selectively placed in contact with a fixed contact or other movable contacts to "make" or "break" current. Of course, the use of two breaks increases the effective contact area, i.e. the area in which the fixed contact touches the moving contact and thus increases switching capacity. U.S. Pat. No. 3,632,935 is representative of patents disclosing a two-break contact rotatable about a center thereof.
Further the use of movable contacts in pairs is well known. As disclosed in U.S. Pat. No. 3,632,935, the use of parallel movable contacts in combination with a single fixed blade or contact generates electro-motive force (EMF) that tends to pull the movable contacts towards one another (a force of attraction) and overcome the tendency of a movable contact to repel a fixed contact (a force of repulsion). In particular, the current flowing through the point of contact between fixed and movable contacts generates a force of repulsion at that point (technically known as "Crowding Effect"). Also, the entire current carrying area of a pair of movable contacts generates a force of attraction therebetween that can be used to overcome the force of repulsion to thereby avoid "popping", i.e. separation of the contacts due to EMF which results in damage to the contacts and failure of the switch. However, it is difficult to implement pairs of contacts that are rotatable about a central portion in rotary switches having more than two breaks. In particular, in order to mount plural contact pairs (which are required for more than two breaks) on the same rotating member, it is necessary to offset the contact pairs to be in different planes to avoid mechanical interference between the contact pairs and to avoid electrical communication between the contact pairs. This increases the required dimensions of the switch.
The "S" Type Fused Combination Switches" sold by MEM SANTON SWITCHGEAR implements a four break rotary switch in a relatively small package by using four single knife contacts on a rotating member and fixed U shaped contacts having two extending portions on respective sides of the knife contacts disposed around the rotating member. However, the relatively small dimensions of the extending portions are not adequate to generate the high attractive forces necessary to avoid popping at high currents. Further, the size and configuration of the fixed contacts does not permit the extending portions to be easily flexed towards each other and thus even if a high force of attraction was generated, the contacts would not "grip" the movable contact and thus are not as stable as true contact pairs. Extending the size of the fixed contacts would increase the size of the device.
The complex interaction between mechanical and electrical forces in a rotary switch have rendered it difficult to implement four breaks in a compact design. Also, known four break rotary switches have relatively unstable contacts or are unduly large.